Molecular characterization of TLR22 and its role in immunological modification of the brood pouch of the lined seahorse, Hippocampus erectus

Abstract

Syngnathidae is a peculiar fish family in which pregnant males incubate developing embryos in a brood pouch which is functionally equivalent to the mammalian uterus. Immunological modifications in the brood pouch during gestation are vital for a successful pregnancy. Previous studies revealed the teleost-specific Toll-like receptor 22 (TLR22) could recognize viruses and bacteria. To evaluate the role of TLR22 in allograft tolerance and antibacterial (and antiviral) protection in the brood pouch, the tlr22 gene was characterized in the lined seahorse (Hippocampus erectus), which has evolved complex and fully enclosed brood pouches. In this study, tlr22 showed specific expression in the brood pouch. Interestingly, expression of tlr22 in the pouch decreased progressively during the course of pregnancy. In addition, the expression of downstream TLR signaling molecules was influenced by pregnancy stages, suggesting that TLR22 and downstream TLR signaling molecules might be involved in immunological tolerance and embryo rejection in the brood pouch during pregnancy. Further investigation of role of TLR22 in immunological protection in brood pouches following the injection of LPS and Poly(I:C) showed that tlr22 expression in the pouch was upregulated after the challenge. In addition, the expression of downstream TLR signaling molecules was altered, indicating that tlr22 and TLR signaling molecules are involved in antiviral and antibacterial immune responses in the brood pouch. Taken together, our results improve the understanding of immunological modifications in the brood pouch and may provide new insights into immune regulation during male pregnancy in syngnathids.